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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

In Situ Monitoring of Individual Plasmonic Nanoparticles Resolves Multistep Nanoscale Sulfidation Reactions Hidden by Ensemble-Average Youngchan Park, Hyuncheol Oh, Jiseong Park, Woong Choi, Hyein Ryu, Daeha Seo, and Hyunjoon Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05630 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

In Situ Monitoring of Individual Plasmonic Nanoparticles Resolves Multistep Nanoscale Sulfidation Reactions Hidden by EnsembleAverage

Youngchan Park†, Hyuncheol Oh†, Jiseong Park ‡, Woong Choi†, Hyein Ryu†, Daeha Seo*,‡ and Hyunjoon Song*,†

†Department

of Chemistry, Korea Advanced Institute of Science and Technology, 291

Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea, and ‡Department of Emerging Materials Science, Daegu-Gyeongbuk Institute of Science and Technology, Daegu, 42988, Republic of Korea

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ABSTRACT

The generation of complex nanostructures to obtain novel characteristics and improved performance has been achieved by coupling multiple nanoscale reactions. Because reactions at nanometer scale directly govern the morphology of nanostructures, understanding the reaction mechanism is critical to precisely control the morphology and, eventually, the physicochemical properties of the materials. However, because of the ensemble-average effect, investigating the reaction mechanism at the bulk level does not provide sufficient information. In this study, we investigated the overall sulfidation reaction mechanism that occurred on individual silver nanocubes in real time at high temperature. Using the single-particle dark-field imaging technique, three discrete steps of the sulfidation reaction were clearly resolved in the profiles of plasmon peak shift and intensity change of individual particles according to time progress: (I) reactant diffusion to silver surface by passing through ligand barrier, (II) silver sulfide formation by C-S bond cleavage of cysteine molecules, and (III) diffusion of silver atoms in silver sulfide layer until the complete 3



formation of silver sulfide. By a combination of simulation and control experiments, physical constants were derived for each step, which is completely hidden in the ensemble measurements. Each individual nanoparticle exhibited a large variation of physical values such as reaction rate constant and diffusivity, mainly resulting from the intrinsic structural heterogeneity. Dark-field microscopy imaging processing based on surface plasmon scattering would be helpful to analyze the reaction kinetics and understand the reaction mechanisms of the numerous multistep nanoscale reactions in real time with high spatial and temporal resolutions under actual reaction conditions.

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INTRODUCTION

The fabrication of complex nanostructures has attracted a great deal of attention because such structures have a potentially wide variation of characteristics and allow additional enhancements of functional properties. Numerous complex structures have been synthesized by coupling multiple reactions either simultaneously1-3 or in sequence.4,5 Gonzalez et al. developed a route for the production of complex hollow nanostructures by sequential and/or simultaneous action of Galvanic replacement and the Kirkendall effect.6 Min et al. synthesized a series of metal-semiconductor hybrid nanostructures via sequential chalcogenization of silver shells followed by cation exchange.7 We have also prepared double shell hollow nanocubes with metal and semiconductor shells by combination of Galvanic replacement, sulfidation, and cation exchange.8 In these examples, the nanoscale reactions directly altered the morphology of the nanostructures, and each reaction step was closely interwound with the others to generate the eventual morphology. Consequently, understanding the

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mechanism of and relationship between distinct reaction steps is vital to precisely tailor the complex nanostructures to exhibit the targeted chemical and physical properties.

To carefully analyze the multistep reaction mechanisms, many characterization tools for solid state materials have been employed, such as scanning probe microscopy, X-ray photoelectron spectroscopy, and electron microscopy.9,10 These major tools provide valuable information on reaction states; however, most of the techniques require extreme conditions such as an ultra-high vacuum environment, and such conditions cannot reflect the actual reaction environment. On the other hand, it may be possible to employ surface infrared and Raman spectroscopic techniques in

situ during the reaction but, due to the very low signal intensity, these methods collect ensemble-average signals over macroscopic surfaces. The recent development of special techniques such as surface-enhanced Raman spectroscopy or surfaceenhanced fluorescence have successfully approached the spatial resolution at the single molecule level, and have made it possible t16o observe anomalous behaviors different from those of the ensemble average.11 Apparently, there is still a large

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demand for general techniques to analyze the surface reactions in real time with high spatial and temporal resolution under actual reaction conditions, particularly for nanoscale reactions.

Because it is highly sensitive to the composition and surface structure of nanoparticles and their surroundings, single-particle spectral analysis based on localized surface plasmon resonance (LSPR) scattering of nanoparticles has been applied to chemical analysis.12–15 Each single-particle can play a role as an independent sensor in dark-field microscopy with high signal-to-noise ratio and spatiotemporally high resolved detection. These in situ studies have provided valuable insight into the adsorption kinetics and solid-state transformation dynamics under realistic conditions. However, the spectrograph used in optical microscopy is a timeconsuming tool because only single stationary nanoparticles can be analyzed and sufficient acquisition time (tens of seconds) is required to obtain a good signal-to-noise ratio for each measurement. Therefore, there exists a strong limitation against the

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provision of reliable statistics because a large amount of single-particle data is demanded.

In the present study, we employed a high-resolution digital color camera and darkfield microscopy to probe the plasmonic scattering of individual nanoparticles. Unlike the previously utilized single-particle spectroscopy, in this method the color change of multiple individual nanoparticles can be simultaneously tracked with a time resolution of tens of milliseconds. We applied this method of plasmon imaging analysis to monitor a multistep nanoscale reaction. The sulfidation of silver nanoparticles, with cysteine as the sulfur source, was selected as a model reaction. By taking scattering photographs as a function of reaction time, a series of step-by-step processes, from adsorption of cysteine on the silver surface to complete transformation into silver sulfide, were successfully elucidated. This has helped to provide a better understanding of nanoscale surface reactions at the single-particle level, something that could not be observed in ensemble average measurements.

EXPERIMENTAL AND THEORETICAL METHODS 8


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Chemicals. Cysteine (Aldrich), silver trifluoroacetate (CF3COOAg, 99.99+ %, Aldrich), sodium hydrosulfide nonahydrate (NaSH•9H2O, Aldrich), hydrochloric acid (HCl, 35.0 %, Junsei), poly (vinylpyrrolidone) (PVP, Mw = 55 000, Aldrich), ethylene glycol (EG, 99 %, J.T. Baker), tetrachloroaurate(III) trihydrate (HAuCl4•3H2O, 99.9+ %, Aldrich) were used as received.

Characterization. The Ag/Ag2S nanoparticles were characterized by transmission electron microscopy (TEM) were obtained on Philips Tecnai F20 operated at 200 kV and FEI Tecnai F30 operated at 300 kV at KAIST. The particle dispersion was dropcast on carbon-supported 300 mesh Cu grids (TED PELLAR, INC). SEM images were obtained using a Veiros 460 Fei operated at 10 kV and 13 nA. The UV-visible-NIR absorption spectra were obtained on a Jasco V530 UV/vis spectrophotometer using colloidal suspensions in water.

Imaging dark-field microscopy and data analysis. Dark-field microscopy (Carl Zeiss, Scope A1) equipped with a halogen lamp (12 V, 100 W) was used, which was focused through a dark-field condenser (N.A. = 1.2 - 1.4) with an immersion oil. The Scattered 9



light from the nanoparticles were collected by 20x dark-field microscope object lens (EC Plan-Neofluar, N.A. = 0.5) and the images were captured by a 2584 × 1936-pixel true color video camera (Axiocam MRc 5). From the captured dark-field image, red, green, and blue color information was extracted using Matlab program and the point in the CIE diagram. Data analysis are explained in the supplementary notes (Figures S1 and S2).

Flow cell system for in situ monitoring of silver sulfidaiton. Diluted Ag nanocubes were injected via inlet tube into a flow cell using a syringe pump. Some Ag nanocubes were bound to the surface of the slide glass by nonspecific binding, followed by injecting deionized water so that the nanoparticles were bound enough to be stable for a while. The cysteine (1 mM) aqueous solutions at 55 °C were injected with a flow rate of 0.025 ~ 0.050 mL min-1 into a cell. The DFM images of Ag nanocubes were obtained every 10 min during 6 hr.

Single-particle dark-field spectroscopy. Single particle dark-field spectroscopy. To compare the spectra obtained by imaging analysis from microscopy, single-particle 10


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spectra were acquired with an inverted dark-field optical microscope (Carl Zeiss, Axiover 40) equipped with a CCD camera (ANDOR, NEWTON DU971N) coupled with a monochromator (Dongwoo Optron Co., Ltd., 500i). A 35 W halogen lamp was used as a light source and the light was focused via an oil dark-field condenser (N.A. = 1.21.4), followed by passing samples through a 40x objective lens (N.A. = 0.98) for darkfield microscope. The images were acquired by a 640 × 480-pixel color video camera (SONY, SSC-DC80). FDTD simulations. The interaction between the electrons on the metal surface and the light was calculated theoretical using commercial software, FDTD Solution (Lumerical

Solutions,

Inc.).

The

bulk

experimental

dielectric

functions

of

nanostructures were utilized from Johnson and Christy for silver and Benett for silver. A spectral range from 300 to 900 nm was radiated into a box and the mesh size was fixed to 0.5 nm. The refractive index of the surrounding was set to be 1.33 for water. The model of a silver nanocube was truncated by a radius of 12 nm based on the TEM image.

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Synthesis of silver nanocubes. 5.0 mL of EG was prepared in a 100-mL round bottom flask and heated until 150 ˚C with stirring. Then, 60 μL of 3.0 mM NaSH in EG was injected into the boiling solution. After 2 min upon injection, 0.50 mL of 3.0 mM HCl in EG and 1.3 mL of 20 mg mL-1 PVP in EG were added in order. After another 2 min, 0.40 mL of 280 mM CF3COOAg in EG was added into the mixture, followed by reflux for 90 min and cool down to room temperature. For purification, the mixture was centrifuged with ethanol and dispersed in ethanol (30 mL).

Silver sulfidation in solution at a bulk scale. The silver nanocubes dispersed in ethanol (10 mL, 9.0 μM) were washed by a dispersion/precipitation cycle for several times with ethanol, and re-dispersed in 9.0 mL of distilled water. The silver nanocubes were mixed with aqueous cysteine solution and bubbled with nitrogen gas for 20 min. 1 mL of cysteine solution (0.090 mM, 1 equiv with respect to the Ag nanocubes) was added to the silver nanocubes dispersion. The mixture was heated up to 60 oC, and the air was flowed to the mixture by syringe pump (0.25 mL h-1). The sample was obtained every 5 h, and measure absorption using UV-Vis-NIR spectrometer.

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Size-distribution

analysis

of

colloidal

Ag

nanocubes.

Size-distribution

measurements of colloidal Ag nanocubes were performed using dynamic light scattering (DLS) analysis (Malvern's Zetasizer Nano ZS). The samples dispersed in water were measured at room temperature as our experimental conditions. The average particle size is obtained from three times repeated measurement and calculated from the intensity weighted distribution. The results showed a high reliability with a polydispersity index (PDI) of 0.181.

RESULTS AND DISCUSSION

Dark-field microscopy study of silver sulfidation induced by cysteine. Interaction between a metal surface and sulfur-containing ligands has been the focus of intense attention, particularly in nano-bio applications, due to the potent covalent attachment of sulfur atoms on the noble metal surface and its biocompatible characteristics.16–18 Generally, sulfidation of silver nanoparticles has been carried out in an aqueous medium using hydrogen sulfide19,20 or sodium sulfide.21,22 Only a few studies have been conducted on the silver sulfidation reaction using organosulfur molecules as a 13



sulfur source,16,23–25 and most of those studies have focused on either the stabilization or passivation of silver nanoparticles. In these cases, prolonged heating led to the transformation on the surface of sulfur to silver sulfide by C-S bond cleavage, of which the mechanism was distinct from sulfidation with sulfide ions. Because silver ions migrate outward to react with sulfur, sulfidation reaction proceeds. The extent of the sulfide formation is dependent on the silver particle size and the organosulfur source.26

We selected the sulfidation of silver nanocubes by cysteine as a model reaction to investigate the conjugation between nanoparticles and sulfur-containing biomolecules. Dark-field images of silver nanocubes were obtained as a function of the reaction time (Figure 1). Real-time monitoring has generally been difficult to apply to reactions in aqueous media at elevated temperature.27–29 To maintain the temperature of a flowtype sample cell, we introduced a heating band to cover the input and output tubes (Figure 1a). A syringe pump was used to insert a 1 mM aqueous cysteine solution at 55 °C into the flow cell (0.025 ~ 0.050 mL min-1). Silver nanoparticles are excellent nanoprobes for monitoring the sulfidation reaction due to their high sensitivity to the

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surrounding environment and also the large difference of dielectric constant between Ag and Ag2S in the visible range (0.0530 and 3.1331 at 500 nm, respectively). Because the LSPR peak wavelength is in the visible range, it was possible to use a true color digital camera to detect color changes. Obvious color changes were observed according to the reaction time progress (Figure 1b). The original blue scattered light of silver cubes changed color through green, yellow, and orange to red, with a clear decrease in intensity. After 360 min, the intensity of scattered light decreased to almost nothing. Magnified images of a single nanoparticle at each stage are presented in Figure 1c. We used the Matlab program to convert the RGB color in the dark-field image into spectral information of scattered light in the CIE 1931 colorimetric diagram (Figure S1, Supporting Information). To confirm that the wavelength obtained by image analysis corresponds to the real spectrum, we picked multiple nanoparticles with various colors bound to the glass surface and analyzed them using an optical microscope and a single-particle spectroscope. In every case, the maximum peak wavelength obtained by single-particle spectroscopy matched the wavelength from the

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image analysis (Figure S2, Supporting Information). As a consequence of the converting process, for the single nanoparticle shown in Figure 1c, we obtained graphs of the LSPR shift and intensity versus time (Figure 1d and e). Using single-particle spectroscopy, another single-particle spectrum was obtained and is presented in Figure 1f and g, confirming the similar time trajectories of the LSPR peak shift and the intensity. This analysis process using optical microscopy and a color CCD camera is much more convenient and time-saving than conventional single particle spectroscopy for observing multiple nanoparticles under identical conditions at the same time. In addition, the flow cell system with temperature control unit allows the study of the solvent-, concentration-, and temperature-dependent behaviors of the reaction and consequently makes it possible to obtain critical kinetic parameters such as the reaction rate and activation energy of each reaction step.

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Figure 1. (a) Schematic representation of dark-field microscope with flow-cell and heating unit setup. (b) Dark-field scattering snapshots of wide field of silver 17



nanoparticles with reaction progress from t = 0 min to t = 360 min. Aqueous cysteine (1 mM) solution at 55 oC was injected through the flow cell where silver cubes laid on. (Scale bar represents 10 μm). (c) Magnified scattering images of single-particles as a function of reaction time. Trajectory plot of (d) peak wavelength and (e) intensity of scattered light converted from RGB information of the silver nanoparticle shown in Figure 1c. Trajectory plot of the plasmon (f) peak wavelength and (g) intensity obtained from single-particle spectroscopy. Correlation between morphology evolution and optical property obtained by FDTD calculation. The LSPR peak shift results from the composition and/or morphology change of the nanostructure. We performed finite-difference time-domain (FDTD) calculations to check how these changes influence the LSPR peak shift during the sulfidation of the silver cubes. The simulated model structure is based on transmission electron microscopy (TEM) images (Figures 2, S3 and S4). Ex situ study of the reaction has been achieved using electron microscopy analysis, which provides structural and compositional aspects of the reaction dynamics. The results correspond to the structural evolution at the point when sulfidation is induced by Na2S.32 The original surfactant on the silver cubes, poly(vinyl pyrrolidone), is selectively bound to 18


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the {100} facets; thus, the {111} facets located on the corners are relatively exposed.33 As a consequence, the reactants, cysteine molecules and/or sulfur ions, are preferentially deposited on the corners of the cubes during the initial stage of the reaction. Based on the TEM images, the initial model is set to be a single silver nanocube with a 72 nm edge length and a rounding radius of 16 nm (Figures 2a, b and S5). Silver sulfide has low density compared to silver, which leads to a slight increase of the edge length as the sulfidation progresses. However, while the plasmon band is not very sensitive to such small changes of the edge length, it is highly sensitive to the remaining pure Ag domain, and for this reason the structural model is treated, for simplification, as having the same edge length. With increasing roundness, the silver corners of the cube changed from silver to silver sulfide. The resulting peak wavelengths are plotted in Figure 2c. With decreasing intensity, the maximum peak shifts to the longer wavelength. In each of the simulated models, the maximum peak and the intensity are plotted as a function of moles of sulfur added (Figure 2d). A nearly linear relationship between the peak shift and the moles of silver sulfide is observed;

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however, this relationship is not as strong as that between the maximum peak intensity and the moles of silver sulfide. This nonlinear reduction in intensity is mainly due to the reduction in the size of the silver core and a plasmon damping induced at the interface between silver and silver sulfide according to Mie theory, which show a large difference of dielectric constants.31 Therefore, we took the peak shift into consideration in our analysis of the reaction kinetics; we use the intensity change as supportive data as well hereafter.

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Figure 2. (a) Representative TEM images and (b) structural models for FDTD simulation of sulfidation reaction. Silver and silver sulfide are colored dark grey and grey, respectively. Scale bar represents 30 nm. (c) Corresponding calculated spectral evolutions over the course of the reaction. Based on the TEM images, the edge length of each model is fixed at 72 nm. (d) Wavelength of the maximum peak (red) and its intensity (black) as a function of moles of sulfur atoms, obtained by calculation.

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Imaging analysis of plasmon resonance at single-particle level. Sulfidation of ten individual silver nanoparticles was monitored with dark-field microscopy, and the resulting images were analyzed through a process of converting RGB color information into wavelengths (Figure 3). Particle marked with a red circle deviate significantly from the plasmon properties of typical Ag nanocubes, thus excluded. Before the aqueous cysteine solution was introduced, silver nanocubes were identified as shiny bright blue spots; however, they turned to dim red spots after 360 min (Figure 3a). The particles, numbered and marked with white dot circles, were analyzed individually, as depicted in Figure 3b. Because all particles were closely placed within the interparticle distance of 100 μm, the differences between the ten individual nanoparticles may have resulted from intrinsic variation in size or shape of the nanoparticles and thickness of the ligand shell, but not from any external factor such as the reactant concentration or temperature. Considering the diffusion coefficient of water, 4.3 ✕ 10-9 m2 s-1 at 55 °C, the reactant would reach all the particles at nearly the same time under the flow. Figure 3c illustrates the average time trajectory of the

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ten nanoparticles. This average seems similar to the UV-visible spectra for the bulk level of the sulfidation reaction conducted in a flask (Figure 3d), but the main difference is the induction period, which is not observed in the bulk experiment because of the vigorous stirring during the reaction. Another difference between the single-particle spectra (Figure 3b) and the ensemble average plot (Figure 3c and d) is the slope of the peak shift versus time. In the middle of the reaction, the individual particles show a rapid peak change, whereas the ensemble average has a broad time span, and the bulk reaction shows even slower change. For the representative single-particle spectral change in Figure 3e, we successfully identified three distinct time ranges, clearly distinguished by the LSPR peak and intensity: (I) peak and intensity unchanged, (II) abrupt LSPR peak shift occurred, and (III) intensity suddenly dropped but maximum peak unchanged. We inquire into these three periods to correlate them with actual reaction steps and their mechanistic details.

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Figure 3. (a) Dark-field images of silver nanocubes before and 360 min after sulfidation reaction. Images circled with red dots are not from silver cubes. Time trajectories of scattering peak shifts of (b) ten individual nanoparticles and (c) their average spectrum during sulfidation reaction. Numbers are those of individual particles circled with white dots assigned in (a). (d) LSPR peak shift of bulk reaction carried out in a flask and 24


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observed by UV-visible spectrometer. (e) Representative LSPR peak shift (black, left axis) and intensity change (red, right axis) of particle 1 in (a).

Kinetic analysis of individual reaction steps. Each of the time ranges, I to III, represents a distinct reaction step, which we referred to as Steps I, II, and III, respectively. The peak shift of Steps I and II were simulated by sigmoidal function fitting (Figure 4a). In these regions, the graph shows an induction period followed by an abrupt increase, which ideally matches an “S” shaped curve with finite limits at both negative and positive infinity. This sigmoidal function has been employed in various reactions such as corrosion, autocatalysis, and metal photodissolution.34–36 For Step III, the peak wavelength is not constant, but the shift rate is extremely slow to reach the longer wavelength, indicating reaction kinetics different from those of Steps I and II. In this region, the scattering wavelength is over 600 nm and is too dim, making it difficult to analyze by means of the color converting process. Instead, the intensity change is used to carry out fitting with an exponential decay function (Figure 4b).

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Step I: Induction period by diffusion through ligand barrier. The induction period is clearly observed for all particles, although the shift onset varies from particle to particle. The induction period is the time required to accumulate a sufficient concentration of reactant around the nanoparticle surface to trigger the sulfidation reaction. Figure 4a and b show time trajectories of the peak shift and intensity change during sulfidation using 1 mM cysteine at 55 °C. According to the fitting, average induction period was estimated to be 110 ± 25 min for the ten individual particles (purple, Figures 4c and S7, Supporting Information). To obtain further insight into the origin of the induction period, we carried out the sulfidation reaction with a distinct sulfur source and thickness of the surface ligand (Figure 4d and e). When sodium sulfide under the same concentration and temperature was used instead of cysteine as the sulfur source, the reaction occurred too fast to monitor, and so the reaction condition was changed to 1 µM of the reactant at room temperature. In this case, the induction period was significantly reduced (2.2 ± 0.5 min) compared to the reaction with cysteine (red, Figure 4c and f). This large difference is attributed to the extent of reactants accessible

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to the surface of the silver nanoparticle from the bulk solution. The sulfide ion has a small radius (1.7 Å) compared to that of cysteine (~7 Å), giving it four times higher diffusivity according to the Einstein-Stokes relationship. Moreover, unlike the sulfide ion, the cysteine molecule requires a certain orientation if it is to have an effective collision on the surface. To correlate the diffusivity with the induction time, the ligand (or surfactant) on the original silver cube, poly(vinyl pyrrolidone) (PVP), should be considered to form a high energy barrier to induce the actual surface reaction. Jain et

al. reported that the ligand layer behaved as an energy barrier against the transport of incoming/outgoing reactive ions.37 To corroborate that the ligand shell served as an energy barrier, we investigated the reaction under a thicker ligand layer so that cysteine molecules have much more difficulty accessing the silver surface, resulting in an expansion of the induction time. Figure 4g shows the results of monitoring cysteine-induced sulfidation performed at 55 °C with a thick PVP ligand shell obtained by eliminating the washing process. The induction time became longer at 147 ± 22 min (blue, Figure 4c). As a result, the induction period is found to be determined by

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the diffusion of cysteine molecules from the bulk to the surface of the silver cubes through the ligand shell. Interestingly, when silver cubes have a thick PVP layer, exponential decay in intensity does not occur. This is because the initial diffusion of cysteine molecules occurs through the PVP layer, but the continuous coordination of cysteine molecules blocks the surface and prevents their additional access to the further reaction. According to the diffusion coefficient D = 2/6t, where is the average ligand layer and t is the induction period for diffusing through the ligand layer, an average diffusion

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Figure 4. (a) LSPR peak change fitted with a sigmoidal function, and (b) intensity change fitted with an exponential decay function. (c) Induction period of silver sulfidation under different conditions of sulfur source and reaction temperature: cysteine, 55 °C (purple); sodium sulfide, 25 °C (red); and cysteine, 55 °C with unwashed samples (blue). The line inside the box represents the median value, with the box enclosing the 25–75 percentiles and the whiskers the entire distribution. Time trajectories of the scattering (d) peak shift and (e) intensity change during sulfidation 29



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using 1 µM aqueous sodium sulfide solution at 25 °C (closed circle) or 1 mM aqueous cysteine solution with unwashed silver cubes at 55 °C (open circle). Each graph is plotted from ten individual nanoparticles. The representative LSPR peak wavelength (black, left axis) and intensity (red, right axis) plots when (f) sodium sulfide or (g) unwashed silver cubes were used. coefficient for cysteine molecules passing through the PVP layer ( = 14 ± 3 nm, derived from TEM images in Figure S6, Supporting Information) is estimated to be 5.1 ✕ 10-21 m2 s-1 and that for sulfide ions is 2.6 ✕ 10

-19

m2 s-1. These values are

comparable to the value of 4.8 ✕ 10-19 m2 s-1 for water molecules passing through a poly(benzylmethacrylate) polymer layer.38 To analyze the induction time, the blocking effect of binding sites cannot be completely ruled out, but the strong interaction between Ag and thiol groups would saturate the surface coverage by cysteine under our experimental conditions. Therefore, the induction time is mainly influenced by diffusion and the physical values we obtained support our explanation on the effect of the PVP layer.37,39 When the PVP ligand layer gets thicker, the induction period becomes longer. Based on this observation, we calculated the PVP layer thickness

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using the diffusion coefficient we had estimated, i.e. 16.3 nm for the unwashed silver cubes. For all individual particles, the diffusion coefficients of different surface sources and the estimated PVP layer thicknesses are listed in Table 1. Every particle shows a distinct induction period, with a 15-20% deviation resulting from the heterogeneity of each structure; the extent of variation is consistent with the dispersion of the ligand layer as measured by TEM.

Table 1. Estimation of diffusion coefficient (D) for (a) cysteine molecules and (b) sodium sulfide ions to diffuse through PVP ligand layers of ten individual silver nanocubes. (c) Estimation of thickness () of thick PVP layer (average value of diffusion coefficient for cysteine is set at 5.1 x 10-21 m2 s-1). (a) Cysteine

(b) Sodium sulfide

(c) Thick PVP

particle #

t (min)

D (x 10 -21 m2 s−1)

t (min)

D (x 10 -19 m2 s−1)

t (min)

(nm)

1

93.8

5.8

2.2

2.5

134.4

15.8

2

87.3

6.2

1.9

2.8

155.8

17.0

3

80.5

6.8

2.0

2.7

149.0

16.6

4

90.8

6.0

2.3

2.4

165.0

17.5

5

117.4

4.6

1.9

2.9

154.2

16.9

31



Page 32 of 59

6

111.8

4.9

1.7

3.2

167.3

17.6

7

113.7

4.8

2.4

2.3

148.2

16.5

8

118.7

4.6

2.5

2.2

154.4

16.9

9

164.9

3.3

2.0

2.7

102.7

13.8

10

125.2

4.3

2.8

2.0

114.7

14.6

Average

110.4

5.1

2.2

2.6

144.6

16.3

Step II: Abrupt peak shift by C-S bond cleavage and silver sulfidation. After the induction period, the LSPR peak abruptly shifted to the longer wavelength (Step II), followed by a slow increase to 600 nm (Step III). The abrupt peak shift is thought to be the beginning of the sulfidation reaction; this is confirmed by TEM images and FDTD calculation. The reaction steps follow pathways similar to those of the Deal and Grove model (linear-parabolic relationship).40,41 This model explains metal film oxidation as dictated by two steps: the reaction rate is linear at the initial time (for a thin film with ~100 nm or less thickness) and parabolic for a thick film due to the slow diffusion between metal and metal oxide. Based on the Deal and Grove model, Step II is sulfidation-limited, and Step III is diffusion-limited.

32


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The reaction mechanism of silver sulfidation by cysteine, in contrast to that by sulfide ion, is composed of two distinct elementary steps. First, organothiol molecules are adsorbed on the silver surface and form a strong S-Ag bond in the presence of oxygen (formula 1). Then, the S-C bond is cleaved to generate silver sulfide (formula 2), as follows:16

4R-SH(aq) + 4Ag(s) + O2(g) → 4R-S-Ag(aq) + 2H2O(l)

(1)

2R-S-Ag(aq) + 2Ag(s) + 2H2O(l)) → 2Ag2S(s) + 4RH + O2(g)

(2)

Sulfur atoms of the organosulfur molecules are favorably adsorbed on the surface of silver cubes, making Ag-S bonds. As a result, the cleavage of S-C bonds is demonstrated by subsequent electron displacement. When forming the Ag-S bond, electron donation from sulfur to silver induces weakening of the C-S bond.42-44 Regardless of the reduction of the energy barrier, C-S bond scission does not readily occur at room temperature, indicating that the activation energy of C-S bond cleavage is higher than that of Ag-S bond formation. Therefore, the reaction temperature should be ramped up to 55 °C. In addition, compared to sulfidation by sulfide ions, the 33



sulfidation rate by cysteine is significantly low even at high temperature, indicating that the rate-determining step is the C-S bond cleavage, not the silver sulfidation step (Figure 5a).

Figure 5. Time trajectories of scattering (a) peak shift and (b) intensity change during sulfidation using 1 mM cysteine at 65 °C (closed circle) and 40 °C (open circle). Each graph is plotted from values of ten individual nanoparticles. (c) Reaction rates of sulfidation under various conditions. Line inside box represents median value with box enclosing 25–75 percentiles and whiskers the entire distribution. Representative graphs of peak wavelength (black, left axis) and intensity (red, right axis) changes by sulfidation at (d) 65 °C and (e) 40 °C. (f) Arrhenius plot of silver sulfidation reactions. 34


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All reaction constants (black open circles) and average values (red open circles) are plotted at three distinct temperatures and fitted to a linear line (red dash).

For a detailed kinetic analysis of this reaction, we performed the reaction at two more distinct temperatures (40 °C and 65 °C, Figure 5b-e). The three temperature points we selected show significant changes in reaction rate, and the temperature ranges in which reaction is too fast or slow to analyze particle plasmon changes are excluded from the experiment. On the basis of the Deal and Grove model, the reaction rate constant for sulfidation can be obtained from the slope of the thickness of the generated sulfide layer versus the reaction time (nm min-1). In our experiment, the peak shift rapidly occurred from 500 nm to 570 nm where, according to the FDTD calculation and TEM images (Figures 2 and S3-S5), the thickness of the sulfidation shell approached 10 nm. From the linear relation between the peak shift and the number of moles of silver sulfide, the shell thickness can eventually be converted to the silver sulfide thickness on the cube surface, where the peak shift of 7 nm

35



Page 36 of 59

corresponds to a 1 nm thickness increase. The slope of the plot in Figure 4a is also converted to the reaction rate constant for sulfidation. Detailed analysis using the Deal and Grove model is in Supporting Information. The resulting peak shift, reaction time, and rate constant of ten individual nanoparticles under the three different temperatures are listed in Table 2. From the average rate constants, the kinetic energy barrier of C-S bond

Table 2. Peak shift, reaction time, and estimated rate constant for cysteine-induced sulfidation at three different temperatures. 40 °C

55 °C

65 °C

particle

peak

reaction

rate

peak

reaction

rate

peak

reaction

rate

#

shift

time

constant

shift

time

constant

shift

time

constant

(nm)

(min)

(nm min-

(nm)

(min)

(nm min-

(nm)

(min)

(nm min-

1)

1)

1)

1

84.0

77.6

0.15

73.8

50.4

0.21

83.5

9.7

1.2

2

73.0

89.7

0.12

87.7

69.1

0.18

83.8

9.6

1.3

3

75.6

77.6

0.14

73.2

38.9

0.27

77.9

8.1

1.4

4

81.1

93.8

0.12

84.1

63.7

0.19

88.5

8.9

1.4

5

76.3

80.1

0.14

80.3

17.7

0.65

80.0

9.7

1.2

6

77.9

113.4

0.10

79.2

31.3

0.36

77.6

9.2

1.2

7

68.0

25.3

0.38

76.2

14.7

0.74

85.5

9.3

1.3

8

72.0

69.8

0.15

66.2

20.5

0.46

77.2

9.7

1.1

36


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9

72.4

91.6

0.11

82.2

6.7

1.76

89.4

9.5

1.3

10

83.9

73.7

0.16

82.6

49.2

0.24

85.9

10.6

1.2

Averag

76.4

79.3

0.16

78.6

36.2

0.51

82.9

9.4

1.3

e

cleavage is estimated according to the Arrhenius equation to be 19.1 kcal mol-1 (R2 = 0.99) (Figure 5f). Sandroff et al. reported that the cleaving energy of the C-S bond for diphenyl sulfide to adsorb on silver is ~ 20 kcal mol-1, in good agreement with our estimation.42 Some individual particles exhibited large reaction rate constants, which may have resulted from the intrinsic structural heterogeneity.45

Step III: Parabolic peak shift by diffusion-limited sulfidation. Before taking into account the interdiffusion between silver and silver sulfide, we have to consider morphology changes of the nanostructure. In the initial period, silver atoms are on the surface, so that the reaction readily takes place to form the Ag-S bond, which is limited by the sulfidation reaction. As the reaction proceeds, however, silver sulfide covers all the surface and forms the shell layer. Therefore, it takes some time for silver atoms to diffuse from the internal region to the surface and generate new Ag-S bonds with 37



approaching cysteine molecules from the solution. Because the size of silver ion (1.3 Å) is smaller than that of sulfur ion (1.7 Å), the diffusion rate of silver ion in the lattice is faster, and can generate Kirkendall holes. As a result, after the formation of complete silver sulfide layers on the surface, the rate determining step changes from sulfidation to inter-diffusion. In the region of Step III, for all sulfidation reaction graphs in Figures 2-5, the maximum peak shows a gradual shift to longer wavelength after the abrupt peak shift in Step II, where the slopes are significantly different.

The intensity in Step III can be simply fitted using an exponential decay function, which gives the half-life, the time required for half of the complete decay (Figure S8, Supporting Information). The diffusion coefficient can be determined as D = r2/6t1/2, where r is half of the edge length of the silver cube, and t1/2 is the half-life. For the 1 mM aqueous cysteine solution, the diffusion coefficient of silver ions in the silver sulfide layer gets higher as the temperature rises; such temperature-dependent diffusivity is well explained by the Arrhenius equation: k = A exp(-Ea/RT), where k is the reaction rate constant, A is a pre-exponential factor, Ea is the activation energy, R

38


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is the gas constant, and T is the absolute temperature. Using this Arrhenius plot, the activation energy of diffusivity of silver ion in silver sulfide was calculated at 12.1 kcal mol-1, which is comparable to the reference value, 11.1 kcal mol-1.46 Even though this general trend follows the Arrhenius plot very well, there is a variation of the diffusivity for each nanocube. Because single crystalline silver cubes become polycrystalline silver sulfide during sulfidation, the diffusion of silver ions can involve a short circuit diffusion mechanism along grain and lattice boundaries, leading to particle-particle variation.

CONCLUSION

We investigated the chemical reaction between cysteine and silver nanoparticles by tracking surface plasmon resonance of individual nanoparticles in situ with dark-field microscopy, followed by RGB analysis using the Matlab program. Color changes of the nanoparticles showed obvious particle-to-particle variation with three discrete reaction steps: diffusion of cysteine through the ligand barrier to the silver surface (Step I), sulfidation reaction including Ag-S bond formation and C-S bond scission at 39



the surface (Step II), and atomic diffusion of silver ions in silver sulfide layers (Step III, Scheme 1). Even though these average reaction parameters show reasonable matches to the values of the bulk reactions, individual nanoparticles show large particle-to-particle variation, reflecting the structural heterogeneity of each particle. Particularly, when obtained by ensemble measurement, the difference in energy barrier in the initial step, originating from the ligand thickness, could lead to misunderstanding of the sulfidation.

Scheme 1. Three distinct reaction steps of silver sulfidation using cysteine. In Step II, gray, white, yellow, blue and red spheres represent carbon, hydrogen, sulfur, nitrogen and oxygen atoms, respectively.

40


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This single-particle level study may expand the use of dark-field microscopy imaging techniques to in situ analysis of numerous surface reactions on plasmonic nanoparticles because our monitoring techniques showed three steps of the general surface reaction, including adsorption, surface reaction, and surface diffusion for further reaction, and individual steps can be analyzed in real time and with various reaction conditions. Our results give evidence that the dark-field microscopic imaging analysis technique can be employed to analyze not only a simple single step reaction but also a sequential multistep reaction. We expect that this color-analysis process based on surface plasmons at single-particle resolution has great potential to provide detailed pictures of mechanisms for a variety of complex heterogeneous reactions.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/jacs.xxxxxxx. The detail description of converting process of color information to the spectra and data analysis. The exact 41



figures of model structure for FDTD calculation. Comparison of spectroscopic data and microscopic data. TEM images of structural evolution during sulfidation. Time trajectories of the peak shifts under various conditions (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-nail: [email protected]. Phone: +82-10-2885-2847.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the KAIST Grand Challenge 30 project. This work was also supported by the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIP) (NRF-2018R1A2B3004096, NRF-2018R1A5A1025208).

42


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TOC Graphic

53



(a) Schematic representation of dark-field microscope with flow-cell and heating unit setup. (b) Dark-field scattering snapshots of wide field of silver nanoparticles with reaction progress from t = 0 min to t = 360 min. Aqueous cysteine (1 mM) solution at 55 oC was injected through the flow cell where silver cubes laid on. (Scale bar represents 10 μm). (c) Magnified scattering images of single-particles as a function of reaction time. Trajectory plot of (d) peak wavelength and (e) intensity of scattered light converted from RGB information of the silver nanoparticle shown in Figure 1c. Trajectory plot of the plasmon (f) peak wavelength and (g) intensity obtained from single-particle spectroscopy. 170x252mm (150 x 150 DPI)


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(a) Representative TEM images and (b) structural models for FDTD simulation of sulfidation reaction. Silver and silver sulfide are colored dark grey and grey, respectively. Scale bar represents 30 nm. (c) Corresponding calculated spectral evolutions over the course of the reaction. Based on the TEM images, the edge length of each model is fixed at 72 nm. (d) Wavelength of the maximum peak (red) and its intensity (black) as a function of moles of sulfur atoms, obtained by calculation. 205x154mm (150 x 150 DPI)



(a) Dark-field images of silver nanocubes before and 360 min after sulfidation reaction. Images circled with red dots are not from silver cubes. Time trajectories of scattering peak shifts of (b) ten individual nanoparticles and (c) their average spectrum during sulfidation reaction. Numbers are those of individual particles circled with white dots assigned in (a). (d) LSPR peak shift of bulk reaction carried out in a flask and observed by UV-visible spectrometer. (e) Representative LSPR peak shift (black, left axis) and intensity change (red, right axis) of particle 1 in (a). 200x197mm (150 x 150 DPI)


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(a) LSPR peak change fitted with a sigmoidal function, and (b) intensity change fitted with an exponential decay function. (c) Induction period of silver sulfidation under different conditions of sulfur source and reaction temperature: cysteine, 55 °C (purple); sodium sulfide, 25 °C (red); and cysteine, 55 °C with unwashed samples (blue). The line inside the box represents the median value, with the box enclosing the 25–75 percentiles and the whiskers the entire distribution. Time trajectories of the scattering (d) peak shift and (e) intensity change during sulfidation using 1 µM aqueous sodium sulfide solution at 25 °C (closed circle) or 1 mM aqueous cysteine solution with unwashed silver cubes at 55 °C (open circle). Each graph is plotted from ten individual nanoparticles. The representative LSPR peak wavelength (black, left axis) and intensity (red, right axis) plots when (f) sodium sulfide or (g) unwashed silver cubes were used. 222x197mm (150 x 150 DPI)



Time trajectories of scattering (a) peak shift and (b) intensity change during sulfidation using 1 mM cysteine at 65 °C (closed circle) and 40 °C (open circle). Each graph is plotted from values of ten individual nanoparticles. (c) Reaction rates of sulfidation under various conditions. Line inside box represents median value with box enclosing 25–75 percentiles and whiskers the entire distribution. Representative graphs of peak wavelength (black, left axis) and intensity (red, right axis) changes by sulfidation at (d) 65 °C and (e) 40 °C. (f) Arrhenius plot of silver sulfidation reactions. All reaction constants (black open circles) and average values (red open circles) are plotted at three distinct temperatures and fitted to a linear line (red dash). 234x128mm (150 x 150 DPI)


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Three distinct reaction steps of silver sulfidation using cysteine. In Step II, gray, white, yellow, blue and red spheres represent carbon, hydrogen, sulfur, nitrogen and oxygen atoms, respectively.


In Situ Monitoring of Individual Plasmonic ... - ACS Publications

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